Typically, sheets of wood product, such as plywood and the like, are cut with various types of saws depending on the desired shape of the cut. For example, table saws or circular saws are generally used to cut along a straight path and band saws or saber saws may be used to cut along curved paths. It is difficult to make a smooth curved cut using a band saw or a saber saw since the saw blade is flat and therefore most naturally follows a straight path. Also, when using a band saw to cut plywood, difficulty arises when trying to maneuver a large sheet of plywood along a desired path into the blade. In the case of using a saber saw, blade breakage commonly occurs due to the twisting action or torque that the blade necessarily goes through as it is forced around a curved path. This twisting action causes the blade to encounter shear forces that it may not be able to withstand, especially if used to cut a curve of small radius.
Router bits are also used to make both straight line and curved line cuts in wood and other similar materials. Router bits withstand much higher shear forces than do the flat blades used in a saber saw, for example. Such bits have generally included one or more straight or helical cutting flutes. Tip flutes have also been used to allow the bit to first cut in the axial direction before cutting in a lateral direction.
One example of a helically fluted wood cutting router bit is shown in U.S. Pat. No. 1,963,611 issued to Brumell et al. in 1934. The bit disclosed by Brumell et al. includes two helical flutes, each flute having a flat land or thread 10 extending rearwardly from a cutting edge 11. The flutes and lands are disclosed as being either right-handed or left-handed. The spiral lands 10 are stated to act as conveyors for clearing the chips from the cuts so that clogging of the cutter and resultant resistance thereto is, to a large extent, eliminated. The patent to Brumell et al. fails to disclose the use of a parabolic grind and instead appears to use a conventional grind.
Another example of a helically fluted router bit for cutting wood is disclosed in U.S. Pat. No. 3,701,188 to Wall et al. In FIG. 2 Wall et al. show a router bit having a single closed left-handed helical flute. The width of the flute spans approximately half the circumference of the body of the router bit. The router bit of Wall et al. is disclosed as being parabolically ground.
Wood router bits such as those described above each have certain disadvantages. Single fluted router bits such as those contemplated by Wall et al. lack strength when formed in relatively small diameters such as 1/8", if used to perform lateral cutting operations on wood sheet products, such as plywood. Wood cutting router bits such as the router bit of Brumell et al. having lands and lacking a parabolic grind tend to be relatively slow cutting. Thus, router bits of this type are also subject to increased breakage due to operators forcing the bit to work faster than its design allows.
Thus, there has been a need for a router bit for cutting wood sheet products, such as wood, plywood, high density chipboard, and the like, which is sufficiently strong that it can be made in a diameter as small as 1/8" without an excessive breakage rate during use, yet can be quickly and easily guided along a cutting path in the sheet product without producing excessive burring along the top of the line of cut.